contrastdetection

Contrast detection, or contrast-detection autofocus (CDAF), is an autofocus method that determines the correct lens position by maximizing image contrast in the scene. The camera analyzes the sharpness or high-frequency content of the image as the lens is moved in small steps, evaluating a focus metric such as edge strength and local contrast. It converges on the focus position that yields the highest measured contrast, using the live view image or a captured frame as the reference.

This approach is common in many digital cameras, smartphones, and video cameras, especially when phase-detection sensors are unavailable or disabled. Because it relies on the data captured by the main image sensor, CDAF does not require a dedicated phase-detection system and can operate with any lens and sensor combination. In practice, the system iteratively searches for the best focus, sometimes using a center region or a broader image area to guide the search.

Advantages of contrast detection include broad applicability and robustness in textured scenes, as well as real-time operation during live view. Limitations include slower performance relative to phase-detection autofocus, particularly in low-contrast or low-light conditions where the focus metric provides weak guidance. It can also hunt or overshoot in difficult lighting, and it provides no direct depth information from a single monocular image.

In many cameras, CDAF is used alone for live view or combined in hybrid autofocus systems that integrate phase-detection data to improve speed while retaining the accuracy of subject contrast analysis.